there are a number of ways to do this. Basically you want to place them into a "deep-backlog". You can do this by somehow tagging the issues you don't immediately care about and then use this "tag" to exclude those issues via the board's filter.
Regardless of the method you choose you then need to edit the underlying filter for the board and exclude based on the method chosen. This all assumes you are using Classic project by the way. :-)
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